Human parainfluenza virus 2 (HPIV‑2) is an enveloped, single‑stranded, negative‑sense RNA virus in the genus Rubulavirus of the family Paramyxoviridae. It is one of four antigenically distinct human parainfluenza viruses and is a major cause of respiratory infections and croup in infants and young children.
Virology & Pathogenesis
The HPIV‑2 genome consists of a nonsegmented negative‑sense RNA molecule of about 15 kb that encodes six common structural proteins: the nucleocapsid (N), phosphoprotein (P), matrix (M), fusion (F), hemagglutinin–neuraminidase (HN) and the large polymerase (L). Unlike influenza viruses, parainfluenza viruses have a single RNA segment and replicate exclusively in the cytoplasm. The HN glycoprotein binds sialic‑acid–bearing glycoconjugates on the surface of ciliated airway epithelial cells, while the F protein mediates fusion of the viral envelope with the host cell membrane. After entry, the viral polymerase transcribes the negative‑sense genome into mRNAs and replicates the genome following the “rule of six,” and progeny virions bud from the cell surface. HPIV‑2 is classified in the genus Rubulavirus, whereas HPIV‑1 and HPIV‑3 belong to the genus Respirovirus. Replication is largely restricted to the upper airway mucosa. Viral replication and host immune responses cause sloughing of epithelium, mucosal edema and increased secretions, narrowing the subglottic airway and leading to the hallmark signs of croup.
Clinical features and prevention
HPIV‑2 circulates seasonally, with outbreaks typically occurring in late autumn and early winter. Along with HPIV‑1 and HPIV‑3, it accounts for roughly three‑quarters of croup cases in young children and contributes significantly to acute respiratory infections in the pediatric population. The illness often begins as a mild upper respiratory infection with rhinorrhea, cough and low‑grade fever. In susceptible toddlers the infection can progress to laryngotracheitis or laryngotracheobronchitis, producing a barking cough, hoarseness and inspiratory stridor. Most cases resolve with supportive care such as humidified air and corticosteroids; nebulised epinephrine may be used for more severe presentations. HPIV‑2 can also cause bronchiolitis and pneumonia, particularly in infants or immunocompromised patients, but systemic spread is uncommon. There is currently no licensed vaccine or specific antiviral therapy. Prevention relies on hand hygiene, respiratory etiquette and isolation of infected individuals to reduce transmission. HPIV‑2 remains an important respiratory pathogen in childhood. Improved understanding of Rubulavirus biology may assist in developing vaccines or antiviral agents for this common cause of croup. Related Terms: Paramyxoviridae, Rubulavirus, Croup, Hemagglutinin–neuraminidase, Fusion protein